Materials inspired by Mother Nature: A 1-pound boat that could float 1,000 pounds

(Nanowerk News) Combining the secrets that enable water striders to walk on water and give wood its lightness and great strength has yielded an amazing new material so buoyant that, in everyday terms, a boat made from 1 pound of the substance could carry five kitchen refrigerators, about 1,000 pounds.

One of the lightest solid substances in the world, which is also sustainable, it was among the topics of a symposium here today at the 243rd National Meeting & Exposition of the American Chemical Society, the world's largest scientific society. The symposium focused on an emerging field called biomimetics, in which scientists literally take inspiration from Mother Nature, probing and adapting biological systems in plants and animals for use in medicine, industry and other fields.

Olli Ikkala, Ph.D., described the new buoyant material, engineered to mimic the water strider's long, thin feet and made from an "aerogel" composed of the tiny nano-fibrils from the cellulose in plants. Aerogels are so light that some of them are denoted as "solid smoke." The nanocellulose aerogels also have remarkable mechanical properties and are flexible.

"These materials have really spectacular properties that could be used in practical ways," said Ikkala. He is with Helsinki University of Technology in Espoo, Finland. Potential applications range from cleaning up oil spills to helping create such products as sensors for detecting environmental pollution, miniaturized military robots, and even children's toys and super-buoyant beach floats.

Ikkala's presentation was among almost two dozen reports in the symposium titled, "Cellulose-Based Biomimetic and Biomedical Materials," that focused on the use of specially processed cellulose in the design and engineering of materials modeled after biological systems. Cellulose consists of long chains of the sugar glucose linked together into a polymer, a natural plastic–like material. Cellulose gives wood its remarkable strength and is the main component of plant stems, leaves and roots. Traditionally, cellulose's main commercial uses have been in producing paper and textiles –– cotton being a pure form of cellulose. But development of a highly processed form of cellulose, termed nanocellulose, has expanded those applications and sparked intense scientific research. Nanocellulose consists of the fibrils of nanoscale diameters so small that 50,000 would fit across the width of the period at the end of this sentence.

"We are in the middle of a Golden Age, in which a clearer understanding of the forms and functions of cellulose architectures in biological systems is promoting the evolution of advanced materials," said Harry Brumer, Ph.D., of Michael Smith Laboratories, University of British Columbia, Vancouver. He was a co-organizer of the symposium with J. Vincent Edwards, Ph.D., a research chemist with the Agricultural Research Service, U.S. Department of Agriculture in New Orleans, Louisiana. "This session on cellulose-based biomimetic and biomedical materials is really very timely due to the sustained and growing interest in the use of cellulose, particularly nanoscale cellulose, in biomaterials."

Ikkala pointed out that cellulose is the most abundant polymer on Earth, a renewable and sustainable raw material that could be used in many new ways. In addition, nanocellulose promises advanced structural materials similar to metals, such as high-tech spun fibers and films.

"It can be of great potential value in helping the world shift to materials that do not require petroleum for manufacture," Ikkala explained. "The use of wood-based cellulose does not influence the food supply or prices, like corn or other crops. We are really delighted to see how cellulose is moving beyond traditional applications, such as paper and textiles, and finding new high-tech applications."

One application was in Ikkala's so-called "nanocellulose carriers" that have such great buoyance. In developing the new material, Ikkala's team turned nanocellulose into an aerogel. Aerogels can be made from a variety of materials, even the silica in beach sand, and some are only a few times denser than air itself. By one estimate, if Michelangelo's famous statue David were made out of an aerogel rather than marble, it would be less than 5 pounds.

The team incorporated into the nanocellulose aerogel features that enable the water strider to walk on water. The material is not only highly buoyant, but is capable of absorbing huge amounts of oil, opening the way for potential use in cleaning up oil spills. The material would float on the surface, absorbing the oil without sinking. Clean-up workers, then, could retrieve it and recover the oil.

Bacterial cellulose (BC) has been reported as the materials in the tissue engineering fields, such as skin, bone, vascular and cartilage tissue engineering. Exploitation of the skin substitutes and modern wound dressing materials by using BC has attracted much attention. A skin tissue repair materials based on BC have been biosynthesized by Gluconacetobacter xylinus. The nano-composites of BC and chitosan form a cohesive gel structure, and the cell toxicity of the composite is excellent. Unlike other groups, which showed more inflammatory behavior, the inflammatory cells of the BC group were mainly polymorph-nuclear and showed few lymphocytes. The BC skin tissue repair material has an obviously curative effect in promoting the healing of epithelial tissue and reducing inflammation. With its superior mechanical properties, and the excellent biocompatibility, these skin tissue repair materials based on BC have great promise and potential for wound healing and very high clinical value.

Nature-based bioactive biomaterials: Current results in development and properties of small-diameter blood vessels made of biodesigned cellulose

To develop novel types of small-diameter blood vessels, the supramolecular fiber network structure and the dimension of tubular hydrogels from biodesigned cellulose (BC) are specifically created. This succeeds directly during its biotechnological fabrication from dextrose using Gluconacetobacter strains and a matrix-reservoir technology. BC tubes with a length of 100 mm and an inner diameter of 4.0-5.0 mm were used to replace the carotid arteries of 10 sheep over a period of 3 month. The grafts have been analyzed using Doppler ultrasonography, extracellular matrix (ECM) stains, and immunostaining. The aim was to get further insights into the interaction of the BC with blood components and living cells, mainly into a) technical feasibility, b) functional in-vivo performance, c) ability of providing a scaffold for the neoformation of a vascular wall, and d) their proinflammatory potential.

Regioselective modification is a crucial frontier in polysaccharide chemistry. Regioselective synthesis of cellulose derivatives can provide critical understanding of structure-property relationships, analytical characteristics, and biological activity as they relate to polysaccharide nanostructure. We report in this presentation on development of novel synthetic methods for the synthesis of regioselectively modified cellulose derivatives, including 6-amino functionalized cellulose derivatives, some of which have considerable promise in drug delivery by enhancing drug solubility, in nucleic acid delivery by formation of polyelectrolyte complexes, and in tissue engineering by interacting with proteins. We will report on both new methods and new materials that have general promise for enhancing control and understanding of cellulose derivative regiochemistry.

Electrospun fibers of regenerated cellulose for biomedical applications: Formation, characterization, and in vitro biocompatibilty of scaffolds

Interest to use cellulosics as polymers in tissue engineering has expanded its potential applications in the biomedical field. Electrospun cellulose acetate, regenerated to cellulose, was transformed into tissue engineering scaffolds with various structures and evaluated as a mimic to native extracellular matrix (ECM) in terms of porosity and fiber alignment. Electrospinning parameters, such as solvent and solution flow rate, were critical variables that influenced the ability of the scaffolding architectures to be maintained during the regeneration process. Laser micro-ablation of the cellulose acetate allowed the controlled design of microporosity to impact cell and nutrient diffusion. Additionally, cytotoxicity tests (minimal essential media elution test and agar overlay), indicate these electrospun scaffolds do not show any morphological changes to cells. These aspects, combined with the toolkit of polysaccharide chemistry for surface modification to enhance bioactivity, suggest that electrospun cellulose acetate is a robust platform for use as a tissue engineering scaffold.

Nanocrystalline cellulose (NCC) is available from the acid-catalyzed degradation of cellulosic materials. NCC is composed of cylindrical crystallites with diameters of ca. 5-10 nm and large aspect ratios. This form of cellulose has intriguing properties, including its ability to form a chiral nematic structure. By using the chiral nematic organization of NCC as a template, we have been able to create highly porous silica films and carbon films with chiral nematic organization.1,2 These materials are iridescent and their structures mimic the shells of jewel beetles. In this paper, I will describe our recent efforts to use NCC to create new materials with interesting optical properties.

Appropriately stabilized cellulose nanocrystal (NCC) suspensions in water form chiral nematic liquid crystalline phases above some critical concentration. In the absence of added electrolye, the chiral nematic pitch of such suspensions is longer than that of visible light. Films prepared by evaporation from the suspensions also often display the characteristic fingerprint patterns characteristic of long-pitch chiral nematic phases, but the pitch values can be shifted into the visible range by adding small quantities of electrolyte to the evaporating suspension. The factors that control the final pitch have been the subject of some confusion. While still not well understood, it is clear that at high nanocrystal concentrations and in solid films, the pitch is not simply a reversible function of nanocrystal concentration. We examine some of the factors that control the pitch and liquid crystal texture during the drying of chiral nematic NCC films.

Optimal wound healing requires a moist and bacteria-free environment. Successful healing depends on the correlation between moisture, the host materials' mechanical properties and the function of active ingredients. We present novel preparation methods for cellulose-based dressings, exhibiting safe antimicrobial activity, while maintaining desired moisture. Safety is assured either by irreversible binding of silver nanoparticles (SND) or a plasma-polymerization (PP) derived surface layer. SNDs were bound in situ by using a starch-based precipitation technique or by sol-gel derived methods, while a mixture of hexane and ammonia was used for deposition of an antimicrobial surface film using PP. PP was performed in a modified GEC cell by utilizing inductive coupled radio-frequency plasma. All materials were characterized by vibrational spectroscopy and XPS, acid orange adsorption studies and potentiometric titrations. Modified cellulose-based materials were proven safe, while in vitro testing revealed simultaneous antimicrobial efficiency against most of typical wounds microorganisms.

Rod-like cellulose nanocrystals (CNCs) offer unique opportunities for the design of therapeutic agents given the established biocompatibility of cellulose and historical use of cellulosic materials in the pharmaceutical industry. Having a large specific surface area, multivalent displays of target-specific ligands can be achieved via covalent conjugation to nanocrystals. In the present study, CNCs produced from sulfuric acid hydrolysis were utilized as a substrate for the conjugation of tyrosine sulfate mimetic ligands. To this end, CNC surface hydroxyl residues were functionalized with small molecule ligands via isothiocyanate chemistry. To increase ligand mobility, oligo(ethylene glycol) molecular spacers were introduced first by activation of CNC surface hydroxyls in aqueous or organic media by epichlorohydrin or 1,1'-carbonyldiimidazole, respectively. The extent of surface functionalization was determined by TGA and XPS. Chemical and topographical information were accessed via ATR-FTIR Spectroscopy and AFM. The potential applications of tyrosine sulfate mimetics are presented and discussed.

Bacterial cellulose (BC) and fibrin are potential materials for artificial blood vessel (BV) applications. Yet each has shortcomings with their mechanical properties when compared to the native BV. To address the shortcomings, BC/fibrin composites with varied compositions were produced. The BC/fibrin composites were further treated with glutaraldehyde in order to crosslink the polymers and allow better match of the mechanical properties with those of native BV. Tensile and viscoelastic properties of the composites were determined from tensile static tests and cyclic creep tests. Glutaraldehyde-treated BC/fibrin composites exhibited comparable tensile strength and modulus with reference small-diameter BV. The cyclic creep test also indicated that glutaraldehyde-treated composites had comparable time-dependent viscoelastic behavior with native BV. A long strain hardening plateau was induced by glutaraldehyde treatment which resembled the stress-strain response of native BV. Covalent bonding between BC and fibrin occurred via glutaraldehyde, affording mechanical properties comparable to the native small BV.

While significant advances have been made in the field of pathogen and chemical detection, these methods often require sophisticated equipment and dedicated laboratory space that is not available in forward operating theaters. To that end, our work focuses on the development of a "Living Membrane" system based on recombinant bacterial strains entrapped in cellulosic membranes, making them resistant to environmental degradation, persistent for short or long periods of time, and housing a built in recording record over time. In this report, we discuss the incorporation of stimuli-sensitive E. coli in to the bacterial cellulose membrane, their distribution, viability, and sensitivity to control stimuli. In addition, we present the construction of a ToxR-based operon for the detection and reporting of specific stimuli. This operon includes the genetic amplifier loop based on the LuxR system. The current report demonstrates the proof of concept of this strategy, and will expand upon continued goals for pathogen and chemical detection.

Nanocellulose is a promising biocompatible hydrogel like nano-biomaterial with potential uses in tissue engineering and regenerative medicine. Biomaterial scaffolds for tissue engineering require precise control of porosity, pore size, and pore interconnectivity. Control of scaffold architecture is crucial to promote cell migration, cell attachment, cell proliferation and cell differentiation. 3D macroporous nanocellulose scaffolds, produced by unique biofabrication process using porogens incorporated in the cultivation step, have shown ability to attract smooth muscle cells, endothelial cells, chondrocytes of various origins, urethral cells and osteoprogenitor cells. We have developed bioprinter which is able to produce 3D porous nanocellulose scaffolds with large size and unique architecture. Surface modifications have been applied to enhance cell adhesion and cell differentiation. In this study we have focused on use of 3D porous Nanocellulose scaffolds for stem cell differentiation into osteogenic and chondral lineages.

Antimicrobial activity of biocomposites based on bacterial cellulose and chitin nanoparticles

Bacteria from the genus Acetobacter represent interesting systems for the design and low-energy fabrication of biomimetic cellulose-based nanocomposites. These microorganisms extruded bacterial cellulose (BC) has the advantage over plant cellulose of being virtually pure and free of any non-cellulosic polymer. Compared to cellulose nanofibers from wood, ribbons that consist of aggregates of BC fibrils can be modified during biosynthesis by the simple addition of water-soluble polymers into the culture medium of the bacterium. Therefore, cultures of Acetobacter represent an ideal biosynthetic system to produce high-strength and functional cellulosic materials with a biomimetic nanostructure. Herein, we report a new series of biocomposites prepared by an in-situ growth process through the addition of chitin nanoparticles into the Acetobacter aceti culture medium. Chitin nanoparticles that were prepared by acid hydrolysis, partially deacetylation, and TEMPO-mediated oxidation are used in this study. The structure and antimicrobial activity of the new BC/chitin nanoparticles biocomposites are investigated.

Production of bacterial cellulose tubes with Gluconacetobacter xylinus by a novel method

Bacterial cellulose shows great promise for applications in the biomedical area. The biopolymer is non-degradable, can be steam sterilized and was shown to be highly biocompatible. Due to low thrombogenicity and high mechanical stability, bacterial cellulose tubes may even be suitable for small diameter blood vessel replacement. We present a novel method utilizing Gluconacetobacter xylinus to produce bacterial cellulose tubes in a reliable and reproducible way. The produced tubes show high mechanical stability. The radial stress at break and the suture pull out strength where evaluated. Tubes were shown to be homogeneous over the length. The wall of the tubes was shown to consist of radially spaced layers, thus providing a high safety margin against burst. Preliminary animal experiments were conducted in pigs. Tubes replaced a part of the arteria mesenterica superior.

Development of cellulose based biointerface for diagnostic and affinity filtration applications

The objective of this study was to develop new strategies for the controlled attachment of antibodies on cellulose and to optimize the surface properties for minimized non-specific binding of target molecules. The surface modification of cellulose was performed by using polysaccharide adsorption and/or chemical functionalization. Consequent attachment of antibodies was achieved either by directly conjugating them onto the functionalized cellulose or by using a specific linker protein that allowed for chemical free attachment strategy. For example, antibodies such as monoclonal anti-hemoglobin and polyclonal anti-human IgG were irreversibly attached to cellulose. Developed platforms were found to be effective on detecting specific antigens with reduced non-specific binding. XPS, QCM-D, SPR and AFM were used to characterize the main chemical, swelling and morphological features of the produced, novel biointerfaces based on the associated, derivatized cellulosic materials. Main results and possible applications for such systems will be discussed.

With the objective to create mechanically adaptive implants, which are sufficiently rigid to allow implantation, but soften thereafter to match the stiffness of their environment, we developed a family of mechanically adaptive polymer nanocomposites. These materials are inspired by the architecture and function of the skin of sea cucumbers, which can change its stiffness on command. Our artificial nanocomposites adopt the architecture of this adaptive tissue and are comprised of soft polymer matrices and rigid cellulose nanofibers. The interactions between the nanofibers are mediated by exposure to water, which causes a dramatic modulus reduction. The new materials were used to probe the hypothesis that the formation of a glial scar, which has been identified as a problem for cortical interfacing, is related to the mechanical mismatch between the soft brain tissue and rigid cortical implants. The fabrication of implants and the results of first in-vivo studies are reported.

Cellulose is the most abundant, biocompatible polymer with low immune response. Unlike synthetic polymers, the degraded products of cellulose scaffolds are not likely to damage the tissues during regeneration. In the present work, we have studied the use of cellulose scaffold for growth of adult stem cells for cartilage tissue engineering. Cellulose scaffolds with varying mechanical properties were used to test their ability to support adult stem cells growth and differentiation. These scaffolds were analysed for levels of viable cell adhesion, and production of various differentiation and transcription markers related to chondrogenesis. The cellulose scaffolds showed good viability and proliferation of stem cells. The stem cells grown on cellulose scaffolds showed chondrogenic gene expression in presence of chondrogenic supplements such as TGF?. Our findings suggest that cellulose scaffolds hold a great potential to be used for stem cells growth and differentiation for biomedical applications, including cartilage tissue engineering.

Plant cells are encased by a cell wall which may display a variety of architectures depending on the particular function (e.g. protection, support, transport, seed dispersal). During growth the cell is surrounded by a primary cell wall which needs to be rigid and compliant at the same time. Flexibility allows cell expansion, rigidity is required to withstand internal and external loads. Although the structures of the individual polymers forming the cell wall are well known; their synthesis, specific arrangement and bonding patterns are not completely understood. The orientation of the stiff cellulose fibrils in the softer matrix is a key factor for mechanical stability and growth anisotropy. To gain further insights we applied a combination of different methods on dark grown Arabidopsis hypocotyls of different ages: Cellulose synthesis was visualized by following the movements of CESA complexes by confocal microscopy. The arrangement of the cellulose fibrils in the wall was determined by SAXS and WAXS. Microtensile tests were performed to analyse their mechanical behaviour. The obtained results may challenge existing models that assume a gradual reorientation of cellulose fibrils during growth.

Bacterial cellulose (BC) has been reported as the materials in the tissue engineering fields, such as skin, bone, vascular and cartilage tissue engineering. Exploitation of the skin substitutes and modern wound dressing materials by using BC has attracted much attention. A skin tissue repair materials based on BC have been biosynthesized by Gluconacetobacter xylinus. The nano-composites of BC and chitosan form a cohesive gel structure, and the cell toxicity of the composite is excellent. Unlike other groups, which showed more inflammatory behavior, the inflammatory cells of the BC group were mainly polymorph-nuclear and showed few lymphocytes. The BC skin tissue repair material has an obviously curative effect in promoting the healing of epithelial tissue and reducing inflammation. With its superior mechanical properties, and the excellent biocompatibility, these skin tissue repair materials based on BC have great promise and potential for wound healing and very high clinical value.